In the production of tubular plastic film, an extrusion die body is employed which has inner and outer die members defining between them an annular space having a longitudinal axis. The outer die member includes an inlet for feeding a stream of resin into the annular space. The inner and outer die members further define an annular chamber which communicates between the inlet and the annular space. A resin stream is supplied from the inlet and divides into two substreams which flow in the annular chamber in opposite directions about the annular space longitudinal axis. These two resin substreams join on the side of the die body opposite the inlet within the annular space to form an annular stream of resin which progresses through the annular space and is finally expressed through an annular orifice of the die body to form the tubular film. The tubular film may be inflated with air to form blown film.
In the area at which the two resin substreams join to form the annular stream of resin, a distinct, so-called "weld line" is formed. This resulting weld line is a source of weakness in the finished film product. Prior efforts aimed at strengthening the weld line or eliminating it as a source of weakness have either not met with complete success or have required rather complicated, and hence expensive, die designs.
It is therefore an objective of the present invention to provide improved method and apparatus for forming tubular film which strengthens the weld line in tubular film.